[clang] [Clang][Cygwin] Remove erroneous _WIN32 define and clean up Cygwin code (PR #138120)

[Martin Storsjö via cfe-commits]

https://github.com/mstorsjo approved this pull request.

I updated the PR description with the details I wanted to have included in the commit message.

https://github.com/llvm/llvm-project/pull/138120